1. Field of the Invention
The present invention relates to detection of an abnormality in a capacitor discharge ignition device that is used for igniting an internal combustion engine.
2. Description of the Related Art
In capacitor discharge ignition devices used for igniting an internal combustion engine, a capacitor for ignition is charged to a prescribed voltage before an ignition time and the resulting charge is released to an ignition primary coil at the ignition time, whereby a high voltage is induced across an ignition secondary coil. Therefore, the capacitor has been charged to the prescribed voltage or more at an instant immediately before the ignition time and the voltage of the capacitor decreases and becomes approximately equal to 0 V after the ignition and before a start of charging for the next ignition. If an abnormality such as a disconnection in a discharge circuit exists from the capacitor to the ignition primary coil, charge is not released to the ignition primary coil and hence the internal combustion engine cannot be ignited. The voltage of the capacitor does not decrease even after the ignition time. On the other hand, if an abnormality exists in a charging circuit, the voltage of the capacitor is not increased to the prescribed voltage and hence cannot cause ignition.
Various means for detecting an abnormality location in ignition circuits are known. Most of those means relate to an induction-type ignition device, that is, they detect an abnormality by detecting an ion current in an ignition plug. For example, JP-A-11-13619 (pages 3–6 and FIGS. 1–4) discloses a technique in which an ion current supply means and an ion current measuring means are provided in an ignition secondary circuit. Ion current values are measured in divisional periods obtained by dividing a period from turning-on of an ignition primary circuit to the end of an ignition discharge. Based on the measurement values, the ignition device judges whether a misfire is due to no specific failure or due to a failure in the input system or the unit.
JP-A-2000-199451 (pages 4 and 5 and FIG. 1) discloses a technique in which an ion current is detected for misfire detection. Although this technique is not intended for detection of an abnormality location, it enables detection of a disconnection in an ignition secondary circuit. To prevent leakage of a high voltage to an ion current detection circuit at the occurrence of a disconnection in the ignition secondary circuit, a Zener diode is provided on the low-voltage side of the ignition secondary circuit. JP-A-2001-132602 (pages 2 and 3 and FIGS. 1, 3, and 4) discloses a technique of judging in which of ignition signal lines of a plurality of cylinders a disconnection has occurred by comparing an ignition signal with a voltage drop across a resistor for limiting an output current of the ignition signal.
In the above conventional devices, the methods of detecting an ion current in an cylinder have an advantage that occurrence/non-occurrence of a discharge for ignition and a combustion state in the cylinder can be detected or measured at the same time. However, those methods have problems that the circuit configuration is complex and the device is expensive for the limited purpose of detecting an abnormality in an ignition circuit because, for example, circuits and elements for measurement need to be added and a peak hold circuit needs to be added because of short duration of a phenomenon.